Police patrol across Westminster Bridge toward the Houses of Parliament in central London on March 23, 2017 after the bridge reopened to traffic following its closure during the March 22 terror attack.

On March 22, Khalid Masood plowed a rented car into pedestrians on Westminster Bridge and stabbed a policeman to death before being shot dead. Immediately after Saturday's London Bridge attack, which Prime Minister Theresa May has said is being treated as a potential terrorist attack, Roberts said he saw six people on the ground.

"I could see six people on the floor not moving. And there were groups of people helping those people on the ground," he said. "After that I heard what sounded like gunfire from Borough Market."

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